In a cellular communication system (like for example the Global System for Mobile communication (GSM) and TETRA (TErrestrial Trunked RAdio)), geographical region is divided into a number of cells each of which is served by a base station. One base station, also termed as Base Transceiver Station (BTS) with a switching infrastructure is referred as a node of the network. The switching infrastructure supporting BTS may be shared with other BTSs. The system infrastructure in a TETRA system is generally referred to as a switching and management infrastructure (SwMI), which substantially contains all of the communication elements apart from the MSs. This includes base transceiver stations (BTSs) connected to a conventional public-switched telephone network (PSTN) through base station controllers (BSCs) and mobile switching centres (MSCs). In geographical terms reference to a node is a reference to an area served by one BTS, and in network terms reference to a node is a reference to one BTS and supporting it infrastructure. A remote unit referred also as subscriber or mobile station (MS) or communication unit is served via a radio communication link by the base station of the cell within which the remote unit is operating.
One of the basic advantages of the cellular communications systems is that subscriber may move from one geographical location to another one while receiving services from the network. To provide seamless service there are regions of overlapping coverage of two base stations. As the subscriber moves from area served by a first base station towards area served by a second base station it enters the region of overlapping coverage. Within the region of overlapping coverage the subscriber changes the serving base station. This is known as cell reselection or handover.
The communication link from a BTS to a subscriber is generally referred to as a downlink communication channel. Conversely, the communication link from a subscriber to a BTS is generally referred to as an up-link communication channel.
To enable communication between two handsets operating in two different cells a fixed network interconnects the base stations. The fixed network is operable to route data between any two base stations and this way allows for communication between these two remote handsets. In addition, the fixed network may comprise gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN). This allows subscribers to communicate with landline telephones and other communication terminals connected by a landline. Additionally the fixed network is adapted to perform functions required for managing a conventional cellular communication network including routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.
The TETRA communications system may be used as a public cellular communication system. However its main target group are organizations or groups such as emergency services. Special functions and services implemented in the TETRA system make this system especially suitable for services like police, emergency, fire rescue or others. One of such features provided by TETRA system, which is especially useful for the emergency services is controlling group calls as well as managing the membership of these groups. Other features and services provided by TETRA include, push-to-talk channel allocation, broadcast calls etc. In addition to trunked mode operation wherein remote units communicate via a base station, TETRA provides for the possibility of communication directly between remote units without participation of the infrastructure. This is known as Direct Mode Operation (DMO).
One of the important issues related to providing communications in cellular networks is mobility management. Main goal of the mobility management is providing and keeping up-to-date the infrastructure with information on location of the subscribers (i.e. in which cell particular subscriber is operating).
Static networks solve the problem by storing location and provisioning information in the HLR. Nodes on the network have tables indicating which HLR contains subscriber provisioning and location information.
Controlling location information of subscribers in static networks, known in the art, is outlined in FIG. i. In a static network 100, a first Home Location Register 118 assigned to a first node 116 contains all configuration data and location information of subscribers 122 and 112 home to the first node 116 and a third HLR 106 assigned to a third node 104 contains all configuration data and location information of subscriber 110. The location information stored in the first HLR 118 (and in any other HLR in the system, i.e. 106 and 128) is a dynamic pointer that can point to any Visitor Location Register (VLR) 108, 120, 130 in the network 100. A first VLR 120 (assigned to the first node 116) keeps information about all subscribers 122 active in a first cell 114 (serviced by the first node 116) and keeps pointers to respective HLRs (i.e. HLRs being home to the subscribers active in the first cell 114). In practice, configuration data of a first subscriber 122 and a second subscriber 112 is kept in the first HLR 118. The first HLR 118 keeps also information about location of the first subscriber 122 and the second subscriber 112, which is active in a third cell 102, and therefore points 132 to a third VLR 108. And the third VLR 108 points 134 to the first HLR 118 as there the configuration data of the second subscriber 112 is stored.
Mobility management scheme used in static networks cannot be implemented in ad-hoc networks, due to the fact that if the HLR function would be assigned to a node of the network, there would be a risk that data stored in that HLR are not be available when such a node is unreachable or even removed from the network at any specific time.
Current ad-hoc networks (i.e. networks which are adapted to be reconfigured during operations) use approaches where the subscribers' provisioning is either distributed replicated) throughout the network, or is contained in some fixed server that is accessed to provide service characteristics. Mobility is handled by ad hoc tables (e.g. the dynamic nature of IP routing tables) where every node in the network either discovers where to route calls for that mobile, or by accessing to the fixed server in a similar way to fixed network operation. If any of this methods cannot be used a scattergun paging is sent out to find location of the mobile.
When a node is moved the users, whose service or location information resides in that node, will either not get service or get a non-complete service from other sites (as their configuration data is not accessible anymore). They may also lose service until the network is updated and (re)configured to the new topology in terms of both location and provisioning information for users. The reconfiguration of the network can take relatively long period of time, and unknown locations may only be solved by widespread paging.
A need therefore exists for a method of controlling location information of subscribers in a communications network, particularly when the network is required to be operational irrespective of reconfiguration processes, wherein the abovementioned disadvantages may be alleviated or overcome.